Quick heated electric heater



Dec. 4, 1962 c. LAcY-HULBERT 3,067,311

QUICK HEATED ELECTRIC HEATER Filed Oct. 2, 1956 48 l Ji x52 IN V EN TOR.

fil La@ -77albef o 2 4 6 e lo t r MINUTES United States Patent O spe/gait QUllCK l-mATED HEATER ("Iyril Lacy-Hulbert, 22 lvietchley Pari: Road, Edgbaston, Birmingham, England Filed Oct. 2, 1956, Ser. No. dime-@3 '7 Claims. till. .M9-2% This invention relates to an electric heater which is adapted? to be supplied temporarily with an abnormal amount of current for quick heating thereof upon initiation of the operation thereof, and has reference more particularly to the provision of the heater with an improved current regulator wherein a conductor of relatively low normal resistance and having a suitable positive temperature co-elicient of resistance is arranged in such relation of heat transfer thereto from the heater that abnormal wattage is supplied to the heater at starting for a sui'licient time to heat the heater quickly to normal operating temperature and is reduced to normal within a safe preheating time.

The invention is applicable to various forms and types of electricheaters, or heating elements as they are usually referred to hereinafter, as for example to heating elements of the sheathed resistor type in which one or more resistance wires, normally in the form of a helical coil, are embedded in a mass of compacted dielectric material which is contained in a metal tube or the like which constitutes the sheath, the dielectric material serving to insulate the coils of wire from one another and from the metal sheath itself, or the heating element may be of a solid hot plate type in which the heating resistance wire is embedded in dielectric material in grooves formed in the underside of a flat plate.

The heating elements of tubular sheathed type are used for the hot plates on electric cookers and, in order that the maximum length of sheath may be accommodated in a given area, the sheath itself, with the resistance mounted therein, is bent around into a llat spiral or similar conguration. The element may be provided with electrical connections at each end or alternatively with connections at one end only, the other end of the sheath being sealed.

It is with elements of this kind which are for use with electric cookers that my invention is particularly concerned though the advantages obtainable from my invention make it also applicable to elements for use in electric kettles, immersion heaters for water storage tanks and similar articles.

A limiting factor in the operation of such elements used as hot plates for electric cookers is that, due to the physical characteristics of the elements, the electrical loading taken by the element at maximum temperature of normal operation, normally about 750 C., must not exceed a predetermined value or otherwise damage may be caused to the element by overheating. The normal safe electrical loading for a hot plate element of approximately 8 inch diameter for normal operation at maximum temperature is of the order of 1.8 kilowatts. Such elements can, however, safely withstand excess wattage or overloading until the temperature thereof reaches the normal temperature rating thereof or the highest temperatureat which it is designed to operate.

With elements as at present in general use the construction is such that the electrical loading taken when the element is cold is substantially the saine as when it is atv maximum temperature and, in order to enable a greater load to be taken by the element when cold and yet not exceed a safe loading at maximum temperature it has been proposed to provide, within the sheath of an element of the closed end type, a control resistance comprising a wire having a high positive temperature coecient of resistance. With such a control wire, which ice is connected in series with the resistance wire, the overall resistance of the heating element and control wire will increase as the temperature rises thereby causing the electrical load taken by the element to fall. Thus an electrical load in excess of that which is safe at its normal high operating temperature, for example a load in excess of the normal load of 1.8 kilowatts, in the above mentioned d inch diameter hot plate element, can be taken by the element when cold so that the time taken to reach maximum temperature is reduced as compared with a conventional element.

it has been found, however, that with an element of the above proposed type where the control wire is contained within the sheath that the control wire rapidly rises in temerature, and thus in resistance, so that the electrical load taken by the element falls toward the safe figure quite rapidly.

The object of the present invention is to provide an improved construction of electric heating element wherein the initial rate of increase in resistance of the control wire is reduced and more effective preheating is accomplishe according to the present invention l provide an electric heating element comprising a main heating element and a control element, said main heating element comprising a wire, or wires, having sufficient specic resistance for the desired heating effect and said control element including a wire having a high positive temperature co-eiiicient of resistance and said control element being so disposed that initial heating thereof is retarded, as for example by heating thereof entirely, or substantially entirely, by radiation from the main heating element.

For example the main element may consist of a length of ordinary resistance wire connected electrically in series with a length of wire having a high positive temperature co-eirlcicnt of resistance, the length of the wire having the high temperature co-eilicient of resistance being of a short length compared to that of the ordinary resistance wire.

lt may be advantageous in seme cases for the main heating element to be of such construction that it has a certain temperature co-eliicient of resistance as well as speci le resistance.

The wire having the high temperature co-ethcient of resistance may be a nickel wire, a suitable nickel alloy wire or an iron or steel wire having a similar temperature co-efticient of resistance.

Alternatively, the main element may be composed of a single length of wire of a known alloy which has the physical properties both of specific resistance suiicient to provide the necessary heating effect and some temperature co-eiiicient of resistance.

The control element is preferably in the form of a tube in which the control wire is mounted in the form of a helical coil embedded in a mass of compacted dielectric material, the tube being mounted below, or within, the convolutions of the main heating element or in other appropriate position of proximity to the main heating element.

ln one form of my invention as applied to an electric heating element for use with the hot plate of an electric cooking stove the complete element includes the main heating element and a control element which are connected together electrically in series. The main heating element may be, for example, of the ended type, that is to say there are terminals at both ends of the sheath which are connected to the control panel of the cooker and, for the sake of simplicity, it will be considered that this heating element contains a single resistance wire which will be wound into the form of a helical coil the wire being embedded in a suitable dielectric material.

The main heating element of this type where only a single resistance wire is provided may be used in conjunction with some suitable energy regulator so as to obtain different heats for the hot plate.

Because of the mass which is necessary to become heated in such embedded resistor heating elements before the exposed surfaces of the heating element become hot enough for the desired cooxing purpose, an excess of heating, sometimes referred to as preheating r as flash heating, is desirable when the heating element is turned on for the cooking operation and in order to achieve such preheating or ilash heating, the control element is connected in series with this main heating element and this control element consists of a wire having a high positive temperature co-eflicient of resistance. With such a wire, as the temperature thereof increases, so the resistance increases and'thus, with a constant voltage, the electrical load taken by the whole element will he less when the element is hot than when the element is cold. For example, a load ot' some 21/2 kilowatts can be applied to such an element of the 8 inch diameter referred to above while it is heating up to its temperature of normal operation and when the element has reached maximum normal operation temperature, about 750 C., the electrical load taken by the element will have fallen to approximately 1.8 kilowatts which is the safe electrical load that can be taken by the element when at maximum operating temperature.

In order to prevent the control element from heating up, and therefore increasing in resistance, too rapidly the control element is so mounted with respect to the main element that it is heated either entirely or substantially entirely by radiation as opposed to conduction.

To achieve this heating by radiation the control element, which consists of a coil of iron or iron alloy wire mounted in dielectric material in a metal tube or sheath, is mounted below the main element. The radiation heating effect upon the control element body is the absolute temperature to the power of four of the main element minus absolute temperature to the power of four of the control element. Thus as the temperature of the main element rises so the radiation eect increases rapidly and as the temperature of the main element approaches maximum heat the control element will itself be being heated very rapidly, both by radiation and self-heating.

This is due to the fact that the flow of current through the control element does not generate much heat when the resistance thereof is cold, and the self generated heat thereof progressively increases as the heating of the control resistance increases, with the result that the initial heating of the control element which occurs when the heating element is turned on for coolring, depends largely upon the transfer of heat thereto from the main heating element, rather than upon the heat supplied by the resistance wire of the control element itself, and the latter does not attain sufficient resistance for substantial self heating until it is heated by the radiated heat from the main heating element.

Such an arrangement, therefore, enables the temperature of the control element to initially lag behind the temperature of the main element and to catch up the temperature of the main element more quickly as the temperature of the latter approaches the desirable maximum. Thus the rate of increase in resistance of the control element is small initially but increases quickly as the main element approaches maximum temperature.

Such an arrangement, therefore, enables the main element to be subjected to an increased electrical load for a suitable time and this load will start falling quickly as the element approaches maximum heat with the result that the time taken to reach normal operation heat is considerably reduced.

lt has, however, been found that with an arrangement as described above it is necessary to provide a physically large control element in order to contain suiiicient sheath area to allow for the wattage dissipation of the control element.

ln order, therefore, to reduce the size of the control element, which may be desirable both from the point of View of economy and from the point of view of space in the hot plate, I have found it sometimes preferable to incorporate in the main heating element a portion of wire having a positive temperature coeilicient of resistance. Thus the entire heater will preferably consist of three wires connected together electrically in series, namely the resistance wire, a portion of wire having a high temperature co-efcient of resistance and the control element which consists wholly of wire having a high temperature co-eihcient of resistance. Although these three wires are connected together electrically in series the control wire is housed in a separate sheath.

Considering the main resistance wire as R1, the portion of wire of high temperature co-e'icient of resistance R2 and the control wire as R3 the following values are chosen as a typical example.

Resistance az Room Temperature Ohms R1 19.4 R2 0.75 a, 1.2

With resistance of these values and a voltage of 240 volts the electrical load taken when such an element is iirst switched on will 'oe about 2,700 watts.

A suitable wire for use as the control wire and also for use in that portion of the main heating element which has a high co-elhcient of resistance is such that its resistance at 750 C. is 7 times its resistance at 0 C. whilst its resistance at 500 C. is five times its resistance at 0 C. Due to the fact that the control element is heated by radiation its temperature will never reach that of the main element and with the main element at 750 C. the temperature of the control element would be about 500 C.

Thus the values for the three resistances when the element is at maximum temperature would be approximately as follows:

R1 19.4 ohms. R2 0.75 7.0=5.25 ohms. R3 1.2 5.0=6.0 ohms.

With these values of resistance the electrical load taken by the element at 750 C. would be approximately 1850 watts.

Alternatively the main heating coil may be made from a known alloy which whilst having suitable specific resistance also has an appreciable temperature co-elcient of resistance and could thus itself constitute the resistances R1 and R2.

lt may be found that the radiation heating alone for the control element will not be suicient to ensure that the electrical load is reduced to a safe figure by the time maximum temperature is achieved and it may therefore be necessary to provide one or more conduction paths between the control element and the main element. These conduction paths can readily be formed, for example, by locally clamping the control element sheath to the main element sheath.

By varying the heating effect on the control element or by altering the value of the resistance of the controlv element R3, the load taken by the whole element at maximum temperature can be varied.

Instead of mounting the control element immediately below the main element it may, if desired, be mounted within the convolutions of the main element.

Instead of providing a single resistance wire in the main element and operating this element in conjunction with an enerUy regulator the main element may consist of two or more resistance wires electrically connected withV a multi-position switch for altering the arrangement of resistances in the circuit so as to achieve differing heats for the hot plate.

The main element instead of having electrical connections at each end may alternatively have all electrical connections at one end the other end being sealed.

Specific examples of heating elements provided with preheating facilities as above described are shown in the accompanying drawing, in which:

FlG. 1 is a side view, mo-stly in longitudinal section, of a combined open ended tubular sheathed heating element with preheater control element thereunder, and with intermediate portions of both elements broken away;

FIG. 2 is a cross sectional view of the structure of FlG. l taken on the line 2-2 thereof;

FIG. 3 is a top view of a tubular sheathed heating element with one end of the tubular sheath permanently closed and the other end open and having a preheater vcontrol element at the opposite sides thereof and with intermediate portions of both elements broken away;

FIG. 4 is lan enlarged cross sectional view of the structure of Fl'G. 3, taken on the 1in-e 3 3 thereof;

FIG. 5 is a top view of another heating element and preheater control element with intermediate portions of the elements broken away;

FIG. 6 is a view of a portion of the length of a tubular sheathed heating element with underlying portion of the length of a preheating control element and showing spaced clamping yof the control element to the heating element for some conductive heating of the control element;

FiG, 7 is a top View showing the application of a preheater controi element between the convolution of a tubular sheathed heater bent in a fiat spiral form in which tubular sheathed heater elements are commonly wound for use as hot plates;

FIG. 8 is a sectional view of one half of a solid hot plate type of heating element having a preheater control element combined therewith in accordance with the present invention; and

FIG. 9 is a chart showing comparative curves of the preheating effect where the controll element is subjected to the radiant heating-effect of the main element and where the control element is not subjected to this radiant heating effect.

Referring first to FIGS. l and 2 of the drawing, the reference numeral 1t? indicates as a whole the heating element and 11 the preheating control element, the former of which is of conventional construction comprising a conventional or standard heating resistance 12 wound in helical form and extending lengthwise in a long metal tube or sheath 13 and electrically insulated from the sheath by a dielectric 14 which may be for example highly compacted magnesium oxide powder, which is commonly used for the purpose and when highly compacted has the characteristic of good heat conductivity. The resistance 12, as is customary, is made of a wire having such specihc resistance to provide the desired intensity of heat.

Terminal wires 1S and 16 are connected within the sheath respectively to the opposite ends of the resistance 12 and project outwardly through plugs 17 of electrical insulating material which close the ends of the sheath 13.

The preheating control element 11, like the hcc-.ting element 1th, comprises a long tube or sheath 1S having a resistance wire 19, preferably in the form of a helix, extending lengthwise thereof and embedded in a dielectric 2th such as compacted magnesium oxide which electrically insulates the resistance 19 from the sheath 1S and the resistance 19 has terminal wires '21 and 22 connected thereto at opposite ends and projecting outwardly through plugs 23 which close the ends of the sheath 13.

The resistance 19 is a wire having a suitable high positive temperature co-eiiicient of resistance, as explained above in the description of the resistance wire which is to be employed for the preheater control element of this in- 6 vention, and such length thereof is employed as isnecessary to obtain the desired preheating control.

This preheating control element 11 is mounted underneath the heating element 10 and spaced therefrom` in any desired manner, substantially as shown in FIGS. 1 and 2, so as to leave therebetween an air gap which provides a field of radiant heat communication and is so arranged that heat is transmitted to the control element 11 from the heater element 10 entirely or almost entirely by radiation only, and not by conduction and may be thus mounted by straps 2li which are welded to and connect the corresponding ends of the elements 10 and 11 to one another.

The terminals 15 and 21 at one end of the combined heater element 10 and preheater control element 11 assembly are connected to current supply conductors 25 and 215, preferably through heater control switch 27 and the terminals 16 and 22 at the other end of the assembly are connected to one another, indicated at 28, so that current is supplied from the conductors 25 and 26 to the resistances 12 and 19 in series.

Thus when the switch 27 is closed, current passes in series through the resistances 12 and 19" and as the high positive temperature resistance 19 of the control element 11 iS cold at the time and the resistances thereof relatively low, abnormal wattage is supplied to the heating resistance 12 to over energize the latter and produce excessive heat for preheating the heating element 10, while at the same time little heat is generated by the resistance 19 itself because of the cold condition thereof.

However, as the heating element 10 becomes heated by the overenergization thereof, heat is transmitted therefrom by radiation to the control element 11 which thereby begins to increase and then progressively increases the temperature of the control resistance 19 and because of the high positive temperature co-efticient of resistance thereof the electrical resistance of the control resistance begins to decrease and this progressively continues to decrease the wattage supplied to the heating resistance 12.

Because of the non-heat conductive relation of the control element 11 with the heater 10, however, and the heat transmission almost entirely by radiation and also because of the mass within which the control resistance `19 is located, the heating of the control resistance 19 is sufficiently retarded to maintain sufcient abnormal wattage supply to the heater resistance 12 for sufficient time for rapid preheating of the heating element 10 to its normal operating temperature.

However, after a safe length of time which is determined by the designed rate of heating of the control element 11 and resistor 19, thereof, the resistance of the control resistor rapidly increases to the resistance value thereof for supplying to the heating resistor '12 the normal Wattage for the heating thereof at the normal operating temperature thereof and thereafter that Wattage is maint red by the control resistor 18.

The combined heating element 10 and control element 11 assembly of FiGS. l and 2, and of the other tubular sheathed assemblies hereafter described, may be in straight lengths or coiled in spirally wound form, somewhat in the manner indicated in FG. 7, for hot plate use and. when thus formed the heating element 10 of FGS. l and 2 will be at the top of the spiral winding and the control element 11 underneath the spirally wound heating element 1t?.

Another arrangement of heating element 29 and control element 3@ is shown in FIGS. 3 and 4, in which said elements are of the same construction as those of FIGS, l and 2, having the resistances thereof embedded in a dielectric in and electrically insulated from metal tubes or sheaths, the heater element sheath 31, however, like that of Tuttle Patent 2,508,552 of May 23, 1950; having one end permanently closed, as indicated at 32, and open at the other end and the resistance being in the form of a loop with a return bend 33 at the closed end 32 of the sheath with two lengths 34 and 3S thereof extending back from the return bend to the open end of the sheath and separated from one another by the dielectric 313 and each provided with a terminal wire, indicated respectively `at 37 and 3S projecting from the open end of the sheath.

The control element St) which is of appropriate length therefor, is bent in a U-shaped form as shown in FIG. 3 so as to have two legs 39 arranged at opposite sides of the heating element 29 and may be mounted in radiant heat transfer relation with respect to the heating element 29 or straps 40 which are secured at suitable intervals to the bottom of the heating element 29, as for example by welding at 41 and have the legs 39 of the control element 3:8 likewise secured thereto.

The control element Sil may be of smaller diameter than the control element Il of FIGS. l and 2, because of the greater length of the control element 3th, and may also have its resistance 42 stretched ont in widely open helical form to appropriately minimize the length thereof, and the resistance l2 has terminal wires i3 and 44 at the opposite ends projecting outwardly through the opposite open ends of the sheath in which it is located.

One terminal 43 of the control element 3@ is connected as at 45 to one terminal 33 of the heater element 29 and current supply conductors te and 47 are connected respectively to the terminal dd and 37 and through a control switch as in FIGS. 1 and 2, so that current passes through the resistances 3d and 35' and 42 in series, and as the control element resistance 42, like the control element resistance 19 of FIGS. l and 2, is made of a wire having a high positive temperature co-eiiicient of resistance and is heated by the heater element 29 in the same manner as the control element l is heated in FIGS. 1 and 2, preheating of the heater element 29 will occur in the same manner as in the construction of FiGS. l and 2.

As hereinafter explained, it may be desirable in some cases, in order to obtain the desired control effect, to locate part of the control resistance in the tubular sheath of the heating element, an example thereof being shown in FIG. in which the heating element is indicated at 48 and the control element at 419 each ot' which comprises a tubular sheath which is permanently closed at one end and open at the other end and having the respective resistances or resistance parts extending lengthwise thereof and embedded in a dielectric.

The control resistance which is of a wire having a high positive temperature co-eicient of resistance has the main portion SiiSL thereof located in the sheath 51 of the control element 49 and an auxiliary portion Sith thereof located in the sheath 52 of lthe heater element i8 along side the heating resistance 53 thereof, as shown in FIG. 5 so that the main portion Stia is heated by radiation by the heater element 4S and the auxiliary portion tb is heated by its proximity to the greater resistance 53 and conductivity of heat thereto through the dielectric from the heating resistance 53.

The portion 50a of the control resistance has a terminal 55 at one end projecting outwardly through the open end of the sheath 51 and has a return conductor 56 extending back therefrom at the closed end of the sheath 5l to the open end of the sheath 51 and provided with a terminal wire 57 projecting from the open end of the Sheath, and the heating resistance 53 in the sheath 52 has a terminal wire 58 projecting therefrom at the open end of the sheath 52 and is connected within the closed end of the sheath 52 to one end of the control resistance portion 50h, as indicated at 59 and the other end of the resistance portion 50h has a terminal wire 6i) connected thereto and projecting through the open end of the sheath 52.

The terminals 55 and dit are connected to one another as indicated at 61 and current supply conductors 62 and 63 are connected respectively to the terminals Sd and S9 so that current passes in series through the control resistance portion Stia inthe control element 49 and through the control resistance portion Sith in the heater sheath 52 and through the heating resistance 53 in the latter sheath.

Thus, in the construction of PEG. 5, the control resistances portion Stia is heated by the heat radiated from the heating element 4.3 to the control element 49 and the control resistance portion 531 is heated directly by conduction from the heating resistance S3 in the sheath 52.

In some cases, as explained hereinbefore, some conductivity of heat may be desirable from the heating element to the control element to supplement the heat transmitted by radiation and for this purpose the control element may be clamped to the heater element at suitable intervals as shown in FIG. 6, in which the reference numeral de indicates a portion of the heater element and a portion or the length of the control element which is clamped at spaced apart places to the control element by straps As further explained hereinbefore the control element may be interposed between convolutio-ns of a heater element which is wound in fiat spiral form to serve as a hot plate, and such interposed arrangement of the control elenient is shown in FIG. 7 in which the reference numeral 67 indicates the spirally wound heater element and 68 the control element, and 69 the connection of the heating and control resistances to one another at corresponding ends of their sheaths and 'itl and 7l indicate current supply conductors which are connected respectively to the heater and control resistances at the other corresponding ends of their sheaths.

As illustrative of the hereinbefore described solid hot plate application of the present invention, I have shown in FIG. 8 a hot plate of conventional form comprising a body 7d of flat circular form having a ilat top surface for supporting a cooking utensil thereon, this body, which is usually a metal casting being formed with a spiral groove in the underside containing a heating resistance 76 which is embedded in electrically insulating heat conductive material 77 which is packed into and hardened in the groove 75 in the customary manner to retain the resistance 76 therein, the resistance 76 being spaced thereby from the body 74 to be safely insulated electrically therefrom.

The resistance 7 6 may be helically coiled wire, as shown in FIG. 7, similar to the heating resistance of the above described constructions, or it may be u strip type resistance of conventional form as preferred.

The control element, which is indicated at 78 is of the same construction as that of the control element 11 of FIGS. l and 2, and may be in spiral or any other desired form, and secured to the underside of the body 74 as for example by a strap 79 so that the coils of the control resistance 73 are spaced sufficiently from the bottom of the body 74 to be heated by radiation from the heated body 74.

The heating resistance 76 is connected in series with the resisttance of the control element 78 and thus the control element of the FIG. 8 construction performs its function in the same manner as the control element of the construction of FIGS. l and 2.

FIG. 9 shows a chart illustrating the comparative performance of a control element which is subjected to radiant heat from the main element, as in the case of the present invention, and a control element which is not subjected to such radiant heat. In both cases, the main heating element and the control element had the same electrical and physical characteristics, but in the curve designated by reference numeral 8l the control element was spaced away from the main heating element, whilst in the case of the curve designated St) the control element was subjected to the radiant heating effect of the main clement. Reference to the chart will show that whereas in the case of the curve 8l the preheating occupies some ten minutes, the provision of a control element subjected to radiant heating from the main element, as shown by the curve 80, enables this preheating time to be reduced by approximately half.

In this chart, the load reduction of the curve 80 has been somewhat amplified for the sake of clarity, so as to better illustrate the advantage obtained with the present invention.

Whilst I have shown and described my invention in a preferred form, and the invention is particularly adaptable to quick heated electric heaters for the reasons eX- plained above, the invention or certain features thereof may be advantageously employed in connection with other electrical heating units and various changes and modifications may be made without departing from the principles of the invention, the scope of which is to be determined by the appended claims.

What I claim is:

1. A heating resistor which is temporarily suppliable with a relatively large amount of current during a short period prior to supply thereof of a smaller normal operating amount, and a control resistor which is electrically connected in series with the heating resistor and conjointly energizable therewith and has a temperature coefiicient of resistance by which the electrical resistivity of the control resistor increases with the temperature of the control resistor and is exercisable through the series connection of the control resistor with the heating resistor to reduce the amount of current supplied to the heating resistor as the electrical resistivity of the control resistor increases, said control resistor normally having at commencement of such conjoint energization a low temperature at which it has low electrical resistivity providing supply of said large amount of current to the heating resistor and said control resistor having an attainable high temperature at which it has high electrical resistivity providing supply of said smaller amount of current to the heating resistor, both resistors being responsive to energization thereof to generate heat and by such responsiveness are operable by conjoint energization thereof during said period to generate in each a separate portion of and furnish conjointly an amount of heat to the control resistor which causes the temperature of the control resistorto increase from said low temperature of low electrical resistivity to said high temperature of high electrical resistivity and reduces the current supply to the heating resistor from said relatively large amount of current to said smaller normal operating amount, said control resistor being operable by said conjoint energization to generate and furnish an aforesaid portion providing a minor amount of said increase of temperature from said high resistivity temperature to said low resistivity temperature and said heating resistor being operable by said conjoint energization to generate and furnish an aforesaid portion providing the major amount of said increase of temperature from said high resistivity temperature to said low resistivity temperature, the control resistor being arranged in a posiiton of communicability of heat thereto from the heating resistor providing a eld of radiant heat transfer by which most of the heat of said portion which provides the major amount of said increase in temperature as aforesaid, is communicated from the heating resistor to the control resistor by radiation through said field.

2. A heating resistor as defined in claim l wherein the heating resistor furnished portion is substantially greater than the control resistor furnished portion.

3. A heating resistor as defined in claim 1 wherein means is provided which delays communication to the control resistor of at least most of the said heating resistor furnished portion of the heat until the latter portion of said period.

4. A heating resistor as defined in claim 1 wherein the control resistor furnishes the control resistor furnished portion of the heat at a rate causing gradual increase in the temperature of the control resistor and at least most of the heating resistor furnished portion of the heat is furnished abruptly to the control resistor.

5. A heating resistor as defined in `claim 1 wherein means is provided which delays communication of the heating resistor furnished portion of the heat during the early portion of said period and communicates it abruptly to the control resistor during the latter portion of said period.

6. A heating resistor as defined in claim 1 wherein the resistors are separated from one another by an air gap and the field of radiant heat transfer is provided by said air gap.

7. A heating resistor as defined in claim 1 wherein each resistor is embedded in a separate mass of electrical insulating material and each mass is contained in a separate tubular sheath.

References Cited in the file of this patent UNITED STATES PATENTS 1,362,227 Clark Dec. 14, 1920 2,135,270 Ashton et al. Nov. l, 1938 2,217,465 Barnsteiner et al Oct. 8, 1940 2,495,461 Kuhn et al Jan. 24, 1950 2,575,131 Lennox Nov. 13, 1951 2,605,380 Bauman et al. July 29, 1952 2,643,317 Tuttle June 23, 1953 2,722,595 Kolb Nov. l, 1955 2,765,391 Shroyer Oct. 2, 1956 2,804,532 Lacy-Hulbert Aug. 27, 1957 

